Wood pulps are generally produced through multistep processes. Initially, logs can be subjected to grinding in which the logs are forced against a rotating abrasive stone which separates the fibers from the log and also the wood cell matrix. In a refining process, wood chips are fed between two metal discs, with at least one disc rotating. In both cases, essentially all of the constituents of wood are retained in the pulp that is eventually produced. Such pulp contains fiber bundles, fiber fragments and whole fibers. A lack of uniformity of pulp and constituents and the presence of lignin in the pulp give it certain desirable qualities, such as yield, paper bulk and opacity as well as good printability. The pulp also has less desirable properties for some paper types, such as low strength, relatively coarse surface and a lack of durability.
Chips to be refined can be destructured and impregnated with chemicals or enzymes prior to further mechanical treatment. This can help increase pulp quality or reduce energy consumption. These methods create slightly different pulps and also vary with the species of wood species, quality of the wood, processing conditions and the amount of energy applied. Various forms exist: thermomechanical pulping (TMP), refiner pulping, stone groundwood pulping, etc.
Chip “destructuring” is usually carried out in the first stage refiner where it occurs in combination with some fiber fibrillation. The difficulty of clearly separating these two steps can lead to an unnecessary increase in energy while no significant gain in pulp properties is obtained. Several pieces of equipment have been developed to overcome these drawbacks. U.S. Pat. No. 5,813,617 of Toma, for example, describes one such device. Other devices incorporate compressive forces along with the destructuring shear forces. These compressive forces along with the accompanied decompression can be used to enhance the penetration of chemicals or enzymes for impregnation prior to refining.
In TMP, steam is added to the chips being refined to facilitate pulping and lower electricity consumption. Steam is also produced during refining and heat recovery systems can help recoup some of the energy cost of the process. The electric motors used to operate these refiners require very large amounts of power. The TMP process generally involves several refining stages to produce a desirable pulp. However, only a small portion of the energy used in each refining stage is actually used to separate and develop the fibers. Screening is used after or between refining stages to separate adequately refined fibers from longer, coarser fibers. These tougher fibers are sent to “rejects” refiners for further development. Depending on the quality of refining, the amount of rejects needing additional refining can and usually is significant.
Woody biomass used in these mechanical pulping processes contains cellulose, hemicelluloses, lignin and extractives in varying amounts throughout the ultrastructure of its fibers. These various components act in conjunction to give these substrates mechanical strength and resistance to degradation. By selectively removing or altering certain components, it is possible to reduce the amount of energy required to separate and refine these fibers. The patent literature describes various approaches using different enzyme mixtures. For example US Patent Publication No. 2005/0000666, of Taylor et al., describes the use of mannanase and xylanase. Certain treatments have been found to significantly impact paper strength properties which have limited their applications. U.S. Pat. No. 5,865,949, of Pere et al., describes a process using an enzyme mixture containing endo-β-glucanase (EG), a limited mannanase and cellobiohydrolase (CBH) activity which reduces the negative effects on paper strength. U.S. Pat. No. 6,099,688, of Pere et al., describes the use of isolated cellobiohydrolase to increase the amount of relative amorphousness of the cellulose within the fibers. This process is said to cause even less damage to paper properties.
International patent publication No. WO 97/40194, of Eachus et al., suggests changing the structure or the composition of the wood by adding to compressed chips fungal or bacterial cultures or products, such as enzymes obtained from them, by means of pressure. The purpose of the compression is to make cracks and fractures in the wood. When the chips are released from the compression, microbes of their products, while the chips expand, are absorbed by the structures of the wood partially by the virtue of negative pressure, partially by the capillary action. The use of lipolytic, proteolytic, linginolytic, cellulolytic and hemicellulolytic enzymes is mentioned. The patent specification describes the absorption of the enzyme preparation Clariant Cartazyme HS™ into the compressed chips after releasing the pressure. Liquid was removed after the treatment, and mechanical pulp was prepared from the chips. In that case, the amount of energy consumed was 7.5% less than in the case of chips that were treated with a buffer only. In another test, the enzyme preparations Clariant Cartazyme NS™ and Sigma porcine pancreas Lipase L-3126 were used. In that case, the amount of energy consumed was 12.5% less than when treated with a buffer only.
A more recent pre-treatment of chips using an enzyme preparation containing cellobiohydrolase and endoglucanase was suggested by Pere in United States Patent Publication No. 2007/0151683. Here again, it was said to be preferable to carry out the enzymatic treatment by compressing the chips and by bringing the compressed chips in a liquid phase into contact with the enzyme composition to absorb the enzyme composition into the chips. The process is said to be useful for reducing the specific energy consumption (SEC) of mechanical pulp and to improve the technical properties of the fibers.